The present invention relates to metal structures in which a metal matrix having a lighter weight and a lower tensile strength at high temperature is reinforced by filaments of a metal present in lower volume fraction but having both higher tensile strength and higher density than that of the matrix. The invention further relates to the reinforcement of lower density metal matrix composites having a niobium titanium base matrix and a higher oxidation resistance, with metal reinforcement having a lower oxidation resistance as well as higher density and higher strength.
The invention additionally relates to body centered cubic metal structures in which a metal matrix having a lower density and a lower tensile strength at high temperature is reinforced by filaments of a metal present in lower volume fraction but having both higher tensile strength and higher density than that of the matrix. Lastly, the invention relates to metal-metal composite structures in which a lower density metal matrix having a niobium titanium base and a higher oxidation resistance is reinforced with denser, but stronger, niobium base metal reinforcing filaments having a lower oxidation resistance.
It is known that niobium base alloys have useful strength in temperature ranges at which nickel and cobalt base superalloys begin to show incipient melting. This incipient melting temperature is in the approximately 2300.degree. to 2400.degree. F. range. The use of the higher melting niobium base metals in advanced jet engine turbine hot sections would allow higher metal temperatures than are currently allowed. Such use of the niobium base alloy materials could permit higher flame temperatures and would also permit production of greater power at greater efficiency. Such greater power production at greater efficiency would be at least in part due to a reduction in cooling air requirements.
The commercially available niobium base alloys have high strength and high density but have very limited oxidation resistance in the range of 1600.degree. F. to 2400.degree. F. Silicide coatings exist which might offer some protection of such alloys at temperatures up to 2400.degree. F., but such silicide coatings are brittle enough that premature failure of the coating could be encountered where the coated part is highly stressed. The commercially available niobium base alloys also have high densities ranging from a low value of 8.6 grams per cubic centimeter for relatively pure niobium to values of about 10 grams per cubic centimeter for the strongest alloys.
Certain alloys having a niobium-titanium base have much lower densities of the range 6-7 grams per cubic centimeter. A group of such alloys are the subject matter of commonly owned U.S. Pats. Nos. 4,956,144; 4,990,308; 5,006,307; 5,019,334; and 5,026,522. Such alloys can be formed into parts which have significantly lower weight than the weight of the presently employed nickel and cobalt superalloys as these superalloys have densities ranging from about 8 to about 9.3 grams per cubic centimeter. One of these patents, U.S. Pat. No. 5,026,522, concerns an alloy having the following composition in atom percent:
______________________________________ Concentration Ingredient Range ______________________________________ niobium balance titanium 35-45% hafnium 10-15% ______________________________________
A number of additional niobium based alloys are also the subject of commonly owned U.S. Patents. These are U.S. Pat. Nos. 4,890,244; 4,931,254; 4,983,356; and 5,000,913. This latter group of alloys has uniquely valuable sets of properties but have densities which are higher than those of the other alloys. Commonly owned U.S. Pat. No. 4,904,546 concerns an alloy system in which a niobium base alloy is protected from environmental attack by a surface coating of an alloy highly resistant to oxidation and other atmospheric attack.
In devising alloy systems for use in aircraft engines the density of the alloys is, of course, a significant factor which often determines whether the alloy is the best available for use in the engine application. The nickel and cobalt based superalloys also have much greater tolerance to oxygen exposure than the commercially available niobium based alloys. The failure of a protective coating on a nickel or cobalt superalloy is a much less catastrophic event than the failure of a protective coating on many of the niobium based alloys and particularly the commercially available niobium based alloys. The oxidation resistance of the niobium based alloys of the above commonly owned patents is intermediate between the resistance of commercial Nb base alloys and that of the Ni- or Co-based superalloys.
While the niobium based alloys of the above commonly owned patents are stronger than wrought nickel or cobalt based superalloys at high temperatures, they are much weaker than cast or directionally solidified nickel or cobalt based superalloys at these higher temperatures. However, for many engine applications, structures formed by wrought sheet fabrication are used, since castings of sheet structures cannot be produced economically in sound form for these applications.
The advantage of use of niobium based structures is evidenced by the fact that the niobium based alloys can withstand 3 ksi for 1000 hours at temperatures of 2100.degree. F. The nickel and cobalt based wrought superalloys, by contrast, can withstand 3 ksi of stress for 1000 hours at only 1700.degree. to 1850.degree. F.
What is highly desirable in general for aircraft engine use is a structure which has a combination of lower density, higher strength at higher temperatures, good ductility at room temperature, and higher oxidation resistance. We have devised metal-metal composite structures which have such a combination of properties.
A number of articles have been written about use of refractory metals in high temperature applications. These articles include the following: